What do you call a shape with eleven straight edges?
Most people will shout “dodecagon” or “pentadecagon” and then stop.
The truth is a little more exotic, and it shows up more often than you think—on dice, in architecture, even in some modern logos.
If you’ve ever stared at a weird‑looking polygon and wondered, “Is that an 11‑gon or something else?Day to day, ” you’re not alone. Let’s clear it up, dig into why it matters, and give you the tools to name any odd‑sided figure without breaking a sweat.
What Is an 11‑Sided Figure Called
In everyday language we call any flat shape with straight sides a polygon. But when you start counting the sides, the naming game begins. An eleven‑sided polygon is officially called an hendecagon.
The Roots of the Word
“Hendeca‑” comes from the Greek hendeka (eleven) and “‑gon” from gōnía (angle). Put them together and you get “eleven‑angle.”
You’ll also see the term undecagon floating around. That’s the Latin‑based cousin: undecim (eleven) + ‑gon. Both are correct; the Greek version is just a bit more common in math circles.
Other Names You Might Hear
- 11‑gon – the shortcut most people use when they don’t want to sound fancy.
- Hendecagonal – the adjective form, as in “a hendecagonal tile.”
- Undecagonal – same idea, just Latin flavor.
So, if you need to sound scholarly, go with hendecagon. If you’re chatting with friends, “11‑gon” works fine.
Why It Matters / Why People Care
You might think, “It’s just a word—why does it matter?”
Real‑World Applications
- Gaming dice – Role‑playing games love polyhedral dice. While the classic d20 gets most of the love, a d11 (a hendecagonal die) pops up in custom sets. Knowing the name helps you search the right forums and find the right molds.
- Architecture & design – Some modern façades use eleven‑sided windows to break up monotony. Architects will list “hendecagonal panels” in their specs; you’ll need the term to understand the blueprint.
- Mathematics education – Teachers use the proper term to teach polygon classification, interior‑angle formulas, and symmetry concepts.
The Short Version Is
If you can name the shape correctly, you’ll look more credible, find the right resources faster, and avoid the awkward “what’s that called again?” moment in a meeting or class Still holds up..
How It Works (or How to Do It)
Naming polygons follows a simple pattern: a numeric prefix + “‑gon.” The trick is remembering the right prefix for 11. Let’s break the process down That's the part that actually makes a difference..
Step 1: Count the Sides
Never skip this. Grab a ruler, count each edge, and confirm you really have eleven.
Step 2: Choose a Language Base
- Greek – Most math texts default to Greek prefixes.
- Latin – Some engineering specs stick with Latin.
For eleven, Greek gives hendeca‑, Latin gives undeci‑ No workaround needed..
Step 3: Attach “‑gon”
Add the suffix, and you’ve got hendecagon or undecagon.
Step 4: Verify the Spelling
It’s easy to typo “hendecagon” as “hendecagone” or “undecagon” as “undecagone.” Double‑check with a quick search if you’re unsure.
Step 5: Use the Correct Form in Context
- Noun: “The floor tiles are shaped like hendecagons.”
- Adjective: “We ordered hendecagonal panels.”
Interior Angle Formula
If you need to calculate angles, the formula for any regular n‑gon works:
[ \text{Interior angle} = \frac{(n-2) \times 180^\circ}{n} ]
Plug in n = 11:
[ \frac{(11-2) \times 180^\circ}{11} = \frac{9 \times 180^\circ}{11} \approx 147.27^\circ ]
So each interior angle in a regular hendecagon is about 147.27 degrees. Handy if you’re designing a tiling pattern.
Symmetry and Construction
A regular hendecagon has:
- 11 lines of reflection symmetry
- 11 rotational symmetries (every 360°/11 ≈ 32.73°)
Constructing a perfect one with just a compass and straightedge is impossible—Gauss proved that only polygons with a number of sides that’s a product of a power of two and distinct Fermat primes are constructible. Eleven isn’t on that list.
Not the most exciting part, but easily the most useful.
But you can approximate it using a protractor or a digital drawing tool. In practice, most designers rely on CAD software that handles the math for you.
Common Mistakes / What Most People Get Wrong
Mistake #1: Mixing Up “Hendecagon” and “Heptagon”
Heptagon = 7 sides. The “he” prefix is easy to confuse, especially when you’re tired.
Mistake #2: Forgetting the “‑gon” Suffix
You’ll see “hendeca” or “undeci” used as shorthand in notes, but they’re not proper nouns. Saying “I need a hendeca for my project” will raise eyebrows Easy to understand, harder to ignore..
Mistake #3: Assuming All 11‑Sided Shapes Are Regular
A regular hendecagon has equal sides and angles. Most real‑world examples are irregular—think of a floor tile that’s slightly stretched to fit a wall. Don’t assume symmetry unless the spec says “regular Small thing, real impact..
Mistake #4: Using the Wrong Language Base in Formal Docs
If a contract uses “undecagonal panel,” swapping it for “hendecagonal panel” could be considered a technical error. Stick with the terminology the original document uses Practical, not theoretical..
Mistake #5: Over‑Complicating the Name
Some people try to invent “eleven‑gon” as “elevenagon.” It sounds cute but isn’t recognized in any standard. Keep it simple: either “hendecagon” or “undecagon,” or the plain “11‑gon Simple, but easy to overlook..
Practical Tips / What Actually Works
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Memorize the Greek prefixes up to 20 – they’re the building blocks for most polygon names Not complicated — just consistent. Which is the point..
- 3: tri‑, 4: tetra‑, 5: penta‑, 6: hexa‑, 7: hepta‑, 8: octa‑, 9: ennea‑, 10: deca‑, 11: hendeca‑, 12: dodeca‑, 13: triskaideca‑, 14: tetrakaideca‑, 15: pentakaideca‑, 16: hexakaideca‑, 17: heptakaideca‑, 18: octakaideca‑, 19: enneakaideca‑, 20: icosa‑.
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Keep a quick reference sheet on your desk or phone. A tiny table with “11 = hendeca / undec” is a lifesaver during design reviews.
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When searching online, try both “hendecagon” and “undecagon.” Some forums prefer one over the other, and you’ll get a fuller set of results It's one of those things that adds up..
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Use a CAD macro that auto‑labels polygons based on side count. Many programs let you write a short script: “if edges = 11 then label = ‘hendecagon’.” Saves you from manual renaming.
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Check the interior angle if you’re doing a tiling layout. Multiply the angle by the number of tiles meeting at a point—if it doesn’t sum to 360°, you’ve got a gap or overlap.
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For dice makers, remember that a regular hendecagonal die will need a slightly recessed center to keep the weight balanced. The geometry isn’t just a name; it affects physics Most people skip this — try not to..
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Teach the term to students or teammates with a visual cue: draw an 11‑gon, label each side, and chant “hendeca‑gon!” The repetition sticks.
FAQ
Q: Is “hendecagon” the same as “undecagon”?
A: Yes. Both refer to an eleven‑sided polygon. “Hendecagon” is Greek‑based; “undecagon” is Latin‑based. Use whichever fits your audience.
Q: Can you draw a perfect hendecagon with a compass?
A: No. Eleven isn’t a constructible number with straightedge and compass alone. You need a protractor or digital tools for an exact regular shape.
Q: How many diagonals does a hendecagon have?
A: The formula is n(n‑3)/2. For n = 11, that’s 11 × 8 / 2 = 44 diagonals.
Q: Are there any famous buildings that use hendecagonal geometry?
A: A few modern museums and pavilions incorporate hendecagonal windows or roof sections to break monotony. Look up the Kunstmuseum Bonn extension—its façade features a subtle 11‑gon motif Practical, not theoretical..
Q: What’s the difference between a regular and irregular hendecagon?
A: Regular means all sides and interior angles are equal. Irregular just means it has eleven sides; lengths and angles can vary Easy to understand, harder to ignore. Still holds up..
Wrapping It Up
So the next time you see an eleven‑sided shape, you can drop “hendecagon” or “undecagon” with confidence. That's why it’s more than a fancy word—it tells engineers, designers, and gamers exactly what they’re dealing with. Plus, remember the quick steps, avoid the common slip‑ups, and you’ll never be stuck wondering how to name that odd polygon again. Happy shaping!
This is where a lot of people lose the thread.
